The core of my unusual earth sheltered design includes 10 precast concrete ribs. I did try to get these done professionally and asked 4 concrete casting companies for quotes, only two got back to me and the average was about $80,000, plus shipping. I asked what they planned to make the forms out of, and both companies said, “plywood”.
Since each of the 10 ribs only includes a few hundred dollars worth of materials and the forms are also relatively cheap to make, I decided to try it myself.
It was definitely more work than I expected, but I was able to keep the cost well below even my estimate by using a lot of the junk wood left over from forming the footings. I think the cost for both forms was less than $300, and then I put in about $200 worth of rebar and 220$ worth of concrete into each. That is less than one 10th the cost of having it done professionally.
The rebar cage was inspected before the pour (you can see the inspectors feet at one point in the video), and passed.
I think the first two ribs turned out well, and there were already some lessons learned, but I will make a few more before I add “Lessons Learned” to my Precast Concrete page.
In the mean time,
Here is the video;
And here is the story;
I like to include this text because it makes it “searchable”. I also try to provide more info than I can provide talking thru the video.
Design
If we get rid of all this peripheral stuff, at the core of this design are radial vaults around a central tower. I need the load from the earth above the vaults to be carried to the footings. I also wanted to keep the vaults small while allowing an open concept living space between them. My solution was to support the vaults with these Euclidean-egg shaped concrete ribs. I chose this shape, with its continuous curve, because of its compressive strength and drawability. The inside end of these ribs will sit on this steel compression ring that will help distribute the load to the tower and steel posts and ultimately into the footings. The egg is tilted so that there is lots of head room near the tower where doorways need to be placed. On the outer edge, an integral column carries the loads directly into large 4’x4′ footing pads.
Eventually, I will span these ribs with steel arches that will form the shape of the vaults and hold up the metal lath that will catch the shotcrete, but I am getting ahead of myself…
First, I need to cast these ribs.
I worked out the forming details in 3D, and it was well worth doing even though I didn’t stick to the plan exactly. The original plan was to layout the wooden forms on a plywood base in the garage (this didn’t quite work out as planned). I used shelf brackets and thin plywood to build the sides of the forms. In the orginal plan, I imagined using 1/4 inch plywood, but In the end, I used multiple layers of 7mm lauan underlayment and a final layer of white board.
The specific rebar layout was designed by my engineer. I really think it is probably “over designed” and working with that #5 rebar is a pain in the butt, but considering how important the strength of these are, I wasn’t going to short it… In fact, based on my own calculations, I actually added additional rebar in the the one place where I felt the engineer had not gone overboard enough… I put an extra two 20ft pieces of #4 rebar along the central spine and belly of the arch. I also integrated 3/8ths inch steel bearing plates at the high end of the arch.
The concrete volume is about 35 cubic feet, which, at about 150lbs per cubic foot, puts the weight of the arch at around 5000 lbs. Maybe the crane operator can tell me the exact weight later.
Form Construction
For practice, I also made a quarter scale model a couple years ago, and practiced drawing out the full scale arch on my driveway.
But this year, it was time to try the real thing. I laid out cheap particle board sheets, screwed them together, coated them with waterproofing and began to construct my curves. This is really a scaled up version of the techniques that old-time graphic designers used to create fonts with tangential curves (before they had computers). The key to tangency is that when two arcs meet, the radial lines where the arcs meet must pass thru the center of both.
Then it was time to cut boards to fit the profile of the arch. This involved tracing them on the floor, then cutting them with a band saw and sanding them. The curves from the band-saw were are not perfect, but I sanded off the main problems and the successive layers should smooth things out.
Then I realized that my particle-wood-base idea was not going to work… humidity and temperature changes during the day just caused too much warping and movement. The waterproofing had been a waste of money and may actually have contributed to the problem because the edges were absorbing moisture and swelling differently to the rest of the boards and this led to curling. Hopefully I can reuse these waterproof boards somewhere else later.
Before removing them, I drilled thru to mark the key centers of the arcs on the floor, then I came back and drilled larger/deeper holes that could hold a nail and work as pivot points so I could quickly reform my egg-arch shapes without redoing the whole Euclidean process.
Then it was time to build up the sides of the forms… I used shelving brackets as planned. Each is supposed to be able to hold 250 lbs. I spaced them less than a ft apart. Even if they had to carry the full weight of the concrete laterally, they should have strength left over. I had to back them with some plywood blocks (not considered in the original design) so my other layers could attach to something.
I needed to make sure that the forms could be easily taken apart. I didn’t want any shifting at the joints, so I used a series of 4 inch offsets with the layers of thin plywood… Where these “separable” layers overlapped, I had to be careful not to glue or screw. Instead, I relied on notched pieces of 2×4 to keep the overlap closed tightly until the lateral load of the concrete was in place to keep it tight. Then I would remove the pegs. (In the video, these were sticking up from the form with yellow tips)
Since I didn’t have the wood base anymore, I eventually needed to attach these form segments directly to the concrete slab with tapcon screws.
The forms for the spandrels (those cut out holes in the ribs) were a bit trickier. I made the series of parts for them with a very carefully planned process of angled passes thru the table saw and router… Then I nailed them all together with my air gun. I had included a fairly significant draft angle so I could get them out of the concrete at the end. This made skinning them with the last white layer a bit more difficult and I had to trim to fit.
I had a similar problem with the tighter curve portion of the rib… I guess my form walls were not perfectly vertical and I had some trouble with the 3rd layer and had to trim it off. After that I used smaller pieces so the angular error didn’t add up.
Eventually, I attached the remaining form pieces to the slab and caulked all the joints… It used up three tubes of silicone.
Then my friend Dan showed up to help out… He took on the job of creating the hangers across the form. These will be used to hang the rebar skeleton so it is positioned in the middle of the concrete. There was also some thought that they might help hold the form together against the pressure of the wet concrete.
Meanwhile, I got started on the Rebar… I had ordered a pallet of pre-bent “Stirrups” and placed just the amount I would need on that pallet in the middle. These were custom ordered and delivered for not much more than the cost of straight rebar. Most places quoted me the price per lb, rather than per bend, so once you pick your supplier, it is pretty easy to estimate costs.
The majority of the rebar was the very thick “#5” rebar. It is not easy to work with, and even more difficult to curve precisely. Dan came up with this idea of clamping the pieces directly to the hangers and forming them in place… It worked very well.
The#3 rebar stirrups need to be threaded on to the long rebar peices ahead of time for easy placement.
Tying rebar is not very fun (time-lapse did not capture many smiles, and this smiling pic above was early in the process. I forget what Dan had just said). You need to bend over a lot and (because I kept not wearing gloves) my hands were pretty cut up by the end of the process… I was glad to be working in doors during all that rain though.
As we neared concrete day, another friend, Aaron, came out to help with the second rib. We used the same process to bend the long pieces and were quite a bit more efficient with this second rib, but it still took me half a day to get all the rebar in and tied.
Pouring the Concrete
Finally, Concrete day arrived. We poured the basement floor and ICF blocks first (another video), and then it was time for the ribs.
Sherri sprayed everything down with form release agent so the concrete wouldn’t stick to the floor or forms. Actually, we used a bug sprayer, but it wasn’t able to spray finely enough (it should be more like a mist) so we wasted a lot of the release agent trying to get coverage. More is not better. The heavier application of release agent was running off the whiteboard instead of coating it. I plan to buy a proper sprayer for next time.
Then the concrete pipe was brought in and things got crazy for a while.
The pipe was very heavy and stiff and generally difficult to move. The pump truck guy took pity on me and stepped in to show me how it was done… Instead of trying to lift it, he was leaning into it. It reminded me of my windsurfing days as a kid. I still never got the hang of moving the concrete pipe effectively, but we got the job done. Next time, I am hopeful that the pump truck operator can park his truck in a slightly different location so that his articulating arm can better access the forms.
We used a concrete vibrator to consolidate everything and it really helped. I had surveyed people on facebook about this and most told me not to worry about it, but these are so critical to the design, but structurally and as visual elements, that I didn’t want to skip on this part. As soon as the vibrator entered the concrete, it liquefied (like sand in an earthquake) and flowed into all the crevasses. Actually, when the vibrator touched the rebar cage, its vibrations were transferred and affected the concrete several feet away. I am glad I didn’t just try to hack it with a sawzall or something like that. I don’t think it would have had the momentum to move all that form or rebar.
The concrete was poured very quickly. I was exhausted and we took a 5 minute break for drinks, etc.
After the concrete setup for a bit, I cut the hanger wires and removed them (along with the hanger boards) and then we troweled over the surface so you don’t see any holes. My wife, Sherri, actually liked the finishing part of the job. We used buckets of water to clean the concrete off the hangers and tools. And, of-course, it was all a race against the setting concrete.
Eventually the crew that had been finishing the basement came in and help with the final edging, etc. This pic is immediately after the chaos, before the cleanup. The wires were hanging the rebar cage. This all cleaned up just fine.
Next Step… we need to take the forms off, and move the ribs out of the garage so we can reset the forms and pour 4 more times.